A series resonant circuit is often applied to a power supply. Please refer to FIG. 1(a), which is a circuit diagram showing a conventional series resonant circuit according to the prior art. In FIG. 1(a), the conventional series resonant circuit includes an input voltage generating circuit, a resonant capacitor Cr, a resonant inductor Lr, an equivalent capacitor Cs, a magnetizing inductor Lm, a transformer Tl, an output rectifier circuit, and an output capacitor Co. The capacitance of the equivalent capacitor Cs is one from the capacitance of a junction capacitor of a rectifier semiconductor in an output terminal of the series resonant circuit and the capacitance of a parasitic capacitor of the transformer Tl which are converted to the primary winding Np of the transformer Tl. An input voltage is generated by the input voltage generating circuit and then frequency-converted by the series resonant circuit, and an output voltage is finally obtained in the output terminal.
The drawback of the conventional series resonant circuit shown in FIG. 1(a) is that a wide range of the operation frequency for the series resonant circuit is needed in order to modulate the output voltage within a wider range of the load variation. However, the highest output frequency of the control chip is limited, and an unpleasant drifted output voltage resulting from the fixed operation frequency of the series resonant circuit with no load may occur.
FIG. 1(b) is a circuit diagram showing two series resonant circuits connected in parallel according to the prior art. FIG. 1(c) is a circuit diagram showing several series resonant circuits connected in parallel according to the prior art. They both have the drawback of the unpleasant drifted output voltage.
To avoid the drawback, a widely-adopted solution is to couple a dummy load, e.g. the dummy load Rdummy shown in FIGS. 1(a)˜1(c), to the output terminal of the series resonant circuit.
In FIG. 1(a), for example, the equivalent capacitor Cs will be charged or discharged by a current when the potential of the input voltage generating circuit varies. The current for charging and discharging the equivalent capacitor Cs must pass through the resonant inductor Lr, so a current difference is generated between the current through the resonant inductor Lr and the current through the magnetizing inductor Lm. After the charging and discharging procedure is finished, the current difference between the current through the resonant inductor Lr and the current through the magnetizing inductor Lm will be transferred from the primary winding Np to the secondary winding Ns and then appear at the load in the output terminal.
To avoid the unpleasant drifted output voltage of the series resonant circuit with no load, the power consumed by the dummy load Rdummy at each operation frequency must be equal to that transferred from the primary winding Np to the secondary winding Ns. Unfortunately, an unbearable high no-load loss is hence obtained.